Market crash- how to define the term?

Market crash, a panic button to everyone, those who are in some way or, others related to the stock market. Always active behind the scene. But, except all those high profiles, very few have the idea, actually when to say a "market crash." Therefore, I was also very eager to know the definition of  "market crash." Here it is to all my fellow friends.

"The term 'Crash" is used almost exclusively to refer to a 20 percent or, more sell-off over a short time on a "Daily" chart and, was widely used only twice in the past 100 yrs! A price action trader looks for a shape and the same crash pattern is common on intraday charts. Since crashes are so common intraday, there is no need to apply the term. Because from a trading perspective, they are just a bear swing with tradeable price action.".

Ref: Trading Price Action Trends, By- AL Brooks. Page-23. para-4.